1. Field of the Invention
The present invention generally relates to perpendicular magnetic recording media and magnetic storage apparatuses, and more particularly to a perpendicular magnetic recording medium having an underlayer that is provided under a recording layer and is made up of a plurality of layers, and to a magnetic storage apparatus having such a perpendicular magnetic recording medium.
2. Description of the Related Art
Recently, due to the high data transfer capability and the large storage capacity of the magnetic storage apparatuses, the magnetic storage apparatuses are no longer used only for personal computers and servers, and there are growing needs to be used for navigation systems for vehicles, portable music players, HDD recorders, portable telephones and the like. For this reason, there are demands to further improve the storage capacity and the recording density of the magnetic storage apparatus.
The magnetic storage apparatus conventionally uses the magnetic recording medium employing the in-plane or longitudinal magnetic recording. The medium noise of the magnetic recording medium employing the longitudinal magnetic recording can be reduced by reducing a remanent magnetization and thickness product tBr and increasing a coercivity Hc of the recording medium. As the remanent magnetization and thickness product tBr is further reduced and the crystal grains of the recording layer become smaller, the remanent magnetization of the recording layer gradually decreases due to the effects of the thermal energy, and the so-called thermal stability deteriorates. In addition, because there is a limit to the magnitude of the recording head field, it is difficult to further increase the coercivity Hc. For these reasons, it is regarded that the further improvement of the recording density is difficult for the magnetic recording medium employing the longitudinal magnetic recording.
In order to further increase the recording density of the magnetic recording medium, there are active research and development of the magnetic recording medium employing the perpendicular magnetic recording, that is, the perpendicular magnetic recording medium. According to the perpendicular magnetic recording, there is an advantage in that the magnitude of the remanent magnetization of the recorded bits stabilizes as the recording density becomes higher, due to the effects of the counter field of the adjacent recorded bits. As a result, the thermal stability is improved in the case of the perpendicular magnetic recording medium.
In addition, the perpendicular magnetic recording medium has a soft magnetic back layer which is made up of a soft magnetic material and is interposed between the substrate and the recording layer. The recording and reproduction of information from and to the perpendicular magnetic recording medium is possible without providing the soft magnetic back layer. However, by using a combination of a single-pole head and the soft magnetic back layer, it is possible to greatly increase the magnetic field generated from the head at the time of the recording, and the generated magnetic field is approximately 1.3 times that of the conventional head for the longitudinal magnetic recording. Accordingly, the coercivity Hc obtainable in the perpendicular magnetic recording medium is higher than that obtainable in the longitudinal magnetic recording medium employing the longitudinal magnetic recording. Moreover, since the soft magnetic back layer sharply draws in the magnetic field generated from the head, the magnetic field gradient becomes small, to reduce the undesirable effects of the spreading of the signal that is written. Therefore, the perpendicular magnetic recording medium has various advantageous features compared to the longitudinal magnetic recording medium.
In order to further improve the recording density of the perpendicular magnetic recording medium, it is essential to reduce the medium noise. In order to reduce the medium noise, it is effective to reduce the orientation or alignment distribution of the axes of easy magnetization of the recording layer. The orientation or alignment distribution of the axes of easy magnetization indicates the extent to which the deviations of the axes of easy magnetization from a direction perpendicular to the substrate surface are distributed.
In the perpendicular magnetic recording medium, a technique has been proposed in a Japanese Laid-Open Patent Application No. 2002-216338, for example, to provide an intermediate layer between the soft magnetic back layer and the recording layer and to control the perpendicular orientation or alignment of the recording layer by the intermediate layer. According to this proposal, a stacked structure, made up of a first intermediate layer made of Cu and a second intermediate layer made mainly of CoCr, is provided between the soft magnetic back layer and the recording layer.
According to the proposal made in the Japanese Laid-Open Patent Application No. 2002-216338, it is expected that the orientation or alignment distribution of the axes of easy magnetization of the recording layer will become satisfactory due to the provision of the stacked structure made up of the first and second intermediate layers. However, because the second intermediate layer is made of a material (ferromagnetic material) having remanent magnetization, this causes the medium noise to increase. Consequently, at the high recording density, there was a problem in that the recording and reproducing characteristics may deteriorate due to the increased medium noise.